1. Field of the Invention
The present invention relates to a fuel cell system and a method for reducing water from a fuel cell system.
2. Description of the Related Art
If excess water is present in a fuel cell and the passages thereof, when a solid polymer fuel cell (hereinafter simply fuel cell) system is stopped, there is the problem of the water content freezing if the system is left for a long period of time below the freezing point. In order to achieve stable operation of a fuel cell system even below the freezing point, it is necessary to remove the water from the passages in the fuel cell system when the fuel cell system is stopped. For this reason, in a fuel cell system that includes passages for gas that circulates, gas is caused to circulate (scavenge) in a passage that includes a gas-liquid separator, so as to reduce the residual water amount. However, if the gas scavenging in the passages of a fuel cell system lasts for a long period of time, the amount of time that the fuel cell system must be stopped is long, so that inconvenience occurs in the use of the fuel cell system. Specifically, because a pump is used to supply scavenging gas and diluted air, there are the problems of a large amount of electrical power being used for the scavenging gas over a long period of time and the noise generated when the pump is driven. Additionally, after the ignition switch is set to off, there is the problem of time being required for processing of the stopping of the fuel cell system, which reduces the value of the fuel cell system as a product.
Additionally, protons (hydrogen ions) within the electrolyte membrane of the fuel cell are moved by water molecules. In operating the fuel cell, in order to achieve proton mobility, it is necessary to moisturize the electrolyte membrane of the fuel cell and the vicinity thereof to achieve smooth movement of protons. For this reason, it is necessary even at below the freezing point to start the fuel cell system smoothly and, in order to achieve a stable operating condition, it is necessary to have some degree of water in the fuel cell.
The above related art does not accommodate short trips, in which there is a relatively short period of operation between the start of electrical generation by the fuel cell and the stopping of electrical generation. In the case of short trips, because electrical generation ends only a short period of time after the starting of electrical generation in the fuel cell, the temperature with the fuel cell system remains low. As a result, there is a small amount of vaporization of water within the fuel cell system, and a large amount of condensed water. Unlike water vapor, condensed water is difficult to scavenge. For this reason, residual water tends to accumulate within the passages of the fuel cell system, so that there is a high possibility of the water not being discharged to outside the passages. In addition, if the gas scavenging time is adjusted in accordance with the electrical generation time, in the case of a short trip, the gas scavenging time becomes short, and there is a great possibility that residual water is not discharged to outside the passages.